Ball valves or ball structures have a valve ball member which is separate from the stem. Stress arises between the stem and ball valve member when the stem is rotated and in turn contacts the ball valve member for rotating the ball valve member against possible high resistance from fluid pressure acting against the ball valve member or from as stop against the ball valve member instead of the stem.
In so-called floating type ball valves in which the ball valve member is not mounted on a fixed bearing for rotation, an elongate slot is normally formed in the ball valve member to receive a lug or key on the inner end of the stem for rotation of the ball valve member. Normally during rotation of the ball member a high stress concentration is obtained at opposite corners of the stem lug against the adjacent flat surfaces or planar of the slot or spline formed in the ball valve member. Under certain high stress conditions and with the ball member made of certain materials, such as ceramics, the ball member may fracture or crack adjacent the slot, and particularly adjacent the outer surface of the ball member at high stress concentrations Ceramic materials are very strong but suddenly fracture or crack once their strength level is exceeded as ceramic does not have a yield point. Ceramics are very rigid with a modulus of elasticity around 30 to 60 million pounds per square inch (psi) as compared with most metal stem materials which have a modulus of elasticity between 14 and 29 million psi. When a stem is engaged to a ball with uniform cross-section throughout its engagement and then a torsional load is applied to the stem, the stem because of its lower modulus of elasticity and smaller cross-section will distort to a greater degree than the ball. This causes the load trasfer point to move higher on the stem. Under extreme conditions, the load is almost entirely carried near the outer periphery of the ball, that is at the point where the stem first engages the ball. In metal ball valves, the material at the outer edge of the ball will eventually yield slightly so that the stem stresses are redistributed more uniformly along the length of the stem to ball engagement. With ceramic balls this doesn't happen. If the ceramic material reaches its breaking strength it simply fractures.